ESOPHAGUS 273 



progressively increase in size toward the lower portion of the tract, 

 where they form the solitary follicles of the intestine. 



The nature of the lining epithelium and the type of the secret- 

 ing glands differs in each succeeding portion of the tract, and must 

 therefore, together with the other peculiarities of the several sub- 

 divisions of the tract, be separately considered. 



THE PHARYNX 



The pharynx may be subdivided, upon histological as well as 

 physiological grounds, into (1) an upper respiratory portion, or 

 naso-pharynx, and (2) a lower portion, oro-pharynx and laryngo- 

 pharynx; only the latter of these properly belongs to the ali- 

 mentary tract. The naso-pharynx has already been described as 

 a part of the respiratory system (see Chapter XIV). 



The mucous membrane of the lower portion of the pharynx is 

 lined by stratified squamous epithelium which rests upon a thick 

 corium of areolar tissue. The tunica propria is well supplied with 

 thin-walled blood vessels and lymphatics, and contains many mucus 

 secreting glands of the tubulo-acinar type whose secreting portions 

 lie deeply embedded in the connective tissue of the muscular coat. 



There is no muscularis mucosse in the mucous membrane of the 

 pharynx ; its place is taken by a layer of connective tissue which 

 is exceedingly rich in longitudinal elastic fibres. This layer lies 

 immediately upon the muscular coat, into which processes of fibro- 

 elastic tissue extend between the muscular bundles; hence this 

 fibro-elastic layer also serves as a submucosa. 



The superficial layer of the corium contains diffuse collections 

 of lymphoid tissue and occasional small lymphatic nodules. 



The muscular coat of the pharynx is formed by its constrictor 

 muscles. Their striated fibres mostly pursue an oblique course. 

 Where these muscles are not immediately attached to the peri- 

 osteum of the v.ertebrae, the pharynx is invested with an outer coat 

 of areolar connective tissue by which it is loosely united to adja- 

 cent organs. 



ESOPHAGUS 



The wall of the esophagus contains the usual four coats : 1, the 

 outer fibrous ; 2, muscular ; 3, submucous ; and 4, mucous. 



THE OUTER FIBROUS COAT envelops the wall of the esoph- 

 agus and unites it to the adjacent organs. It consists of loose 

 fibrous tissue, and contains the blood and lymphatic vessels and 

 nerve trunks which supply the three inner coats. 

 19 



